The present invention relates generally to communication systems and in particular, to adaptive modulation/coding and power allocation in such communication systems.
In order to increase the throughput of a link within a communication system, it has been proposed to utilize multistream transmission techniques with spatial multiplexing. In its simplest sense, multistream transmission can be thought of as the transmission of multiple data streams from a single transmitter source, using multiple transmit antennas, to a single receiver, using multiple receive antennas. Each data stream is transmitted utilizing the same channel resource (e.g., frequency/timeslot/code) and can be distinguished by the receiver due to the fact that the antennas are separated spatially at both the transmitter and the receiver. The receiver combines the signals from the multiple receive antennas to reconstruct the transmitted multistream data. Additionally, multistream methods can be combined with feedback from the receiver to the transmitter so that adjustments can be made for changing channel conditions.
It has been proposed to utilize stream-level adaptive modulation and coding (AMC) and stream-level power control techniques along with multistream transmission to further improve link performance. With stream-level power control (simply referred from now on as xe2x80x98power controlxe2x80x99), the power of a transmitted stream is adjusted to some target power in order to meet a target signal-to-interference-plus-noise ratio, S/(I+N), at the receiver. (S/(I+N) is provided via feedback from the receiver). With stream-level AMC (simply referred from now on as xe2x80x98AMCxe2x80x99), the modulation and coding format of a stream is changed to match the current received signal quality (S/(I+N)). In a system with AMC along with multistream transmission, streams with high S/(I+N) are typically assigned higher-order modulations and high code rates (e.g., 64-QAM with rate-xc2xe convolutional coding), with the modulation-order and/or the code rate decreasing as S/(I+N) decreases. In a system with AMC combined with power control along with multistream transmission, streams with high S/(I+N) are typically assigned higher power along with higher-order modulations and high code rates, with the power level, the modulation-order and/or the code rate decreasing as S/(I+N) decreases. The total available link power is usually distributed across streams in order to match or approximate a water-filling distribution.
Unfortunately, current AMC and power-control methods fail to make efficient use of power-control techniques to allow for the highest combined data throughput over the various streams utilized by the receiver. More particularly, a number of streams may not have sufficient power to support higher code rates while other streams may be utilizing more power than is necessary to support the higher code rates. Accordingly, there is a need for AMC and power allocation which makes make efficient use of power-control techniques to allow for the highest combined data throughput over the various streams utilized by a receiver.